It is known that certain industrial hide treatments, such as tanning, retanning and greasing, are performed in the wet, i.e. by means of immersing the hides in more or less dense water-based, oil-based or grease-based liquids, which contain chemical substances of an organic or inorganic nature and have the function of inhibiting bacterial and enzymatic processes or protecting the hides and giving them a specific aesthetic appearance.
After the abovementioned treatments, the wetted hides must be dried and stretched in order to proceed with the subsequent working steps.
It is also known that the surface area of the hides is a factor of vital importance for the cost-efficiency of a tannery, in that the price of the finished products, for the same final quality, is determined on the basis of this parameter. Therefore, one of the main objectives of tanneries is that of producing hides having the largest footage or surface area possible, in a manner which is compatible with the other qualitative demands.
In the past, in order to perform the stretching operation, traditional straight-edge blades were used, said blades requiring a large amount of effort on the part of the operators and having a limited effectiveness.
This manual operation has gradually been replaced by other automatic operations, generally called “setting out”, which are performed, for example, with wringing machines having spiralled cylinders arranged opposite each other. Alternatively, the hides may be pressed and stretched by means of presses provided with pairs of felt belts which act on the hides during the feeding thereof.
Another method for stretching the hides consists in so-called “pasting”, which envisages a paste being deposited on the coarse side of the wetted hides, said paste having the function of making the hides stick to glass, porcelain or metal plates. The hides treated in this way pass into a tunnel drier until they lose a significant proportion of their moisture. Once dried, the hides are separated and are in the form of flat sheets, ready for finishing and subsequent processing.
Another method of stretching the hides, known in technical jargon as “toggling”, envisages the use of grippers or clamps applied along the edges of the hides and directed radially towards the outside. The clamps are secured to frames which keep the hides extended while they are passed through a tunnel drier or along a path in a controlled atmosphere. Optionally, the frames are provided with plates for supporting the hides in sections which can be separated, as described, for example, in U.S. Pat. No. 2,834,147.
A significant drawback of said known method lies in the fact that the areas affected by the grippers are put under greater stress than the central areas, in such a way that they are irreversibly damaged, giving rise to so-called “bubbles” or blisters due to the separation and creep of the fibres with respect to the skin. Moreover, the thickness of the hides is less in the gripped areas than the central areas and therefore varied stretching and reduction in thickness of the hides occurs, with consequent lower quality of the finished product.
Another known method, described in French application FR-A-2602795, envisages the use of a pair of flat plates on which respective elastically deformable mats, generally made of rubber, are placed. By compressing the hides between the mats arranged opposite each other and applying a longitudinal pulling force to the latter, stretching of the hides by means of friction is achieved. Moreover, hot air, which has the function of drying the hides, is directed between the mats. Unfortunately, the heat used for heating the hides also damages the rubber mats and therefore limits their working life. Moreover, as the working temperature cannot exceed certain maximum values, the drying time increases, reducing the efficiency of the process.
Said known stretching methods have the drawback of subjecting the hides to a mechanical stretching action which is concentrated in particular along the peripheral edge thereof. When acting on said areas, which are the thinnest areas, it is necessary to restrict the magnitude of the stresses in order to avoid breakage. This restricts the increase in the surface area and therefore the efficiency of the method.
Moreover, the stresses are not uniformly distributed and, above all, are not applied towards the centre of the hide, which has the greatest thickness and could therefore allow the greatest stretching.
British patent application GB-A-2236111 describes a method for increasing the total surface area of chrome-tanned or “wet blue” tanned hides, which envisages the moist hides being placed on a flat surface and being subjected to a simultaneous action of longitudinal traction, by means of grippers, and compression, by means of rollers displaced longitudinally with an alternating motion, which is repeated after rotation of the hides through about 90° C. This treatment allows an increase in surface area of about 10%.
However, even this solution, like the preceding solutions, does not eliminate the problem of mechanically stressing the peripheral zones of the hides and not achieving a uniform stretching thereof.
Moreover, the application of this method does not allow the fibres of the hide to be stretched in the region of its central part and therefore the liquids absorbed in this zone are retained for longer than in other zones, increasing the drying times.
A method and a line for stretching hides and other similar products is known from Italian industrial patent application No. VI2001A000220.
In this known method, the wetted hides supplied following a treatment with liquids, such as tanning, retanning, dyeing or the like, are partially dried, for example by means of setting out, and then stretched by means of localised mechanical stresses, which are applied in directions substantially perpendicular to the plane of lie of the hides so as to obtain a predetermined increase in surface area, lastly the hides are partially dried so as to stabilise and heat-fix the stretched hides. This known method is characterized in that during the stretching step, the hides are kept wet with a substantially constant level of moistness, while the mechanical stresses are exerted across the entire surface area of the hides in a substantially uniform manner, without any constraint along their peripheral edges in order to obtain a radial stretching action over their entire surface area.
This method allows the hides to be dried in a substantially uniform manner and results in products of a better quality at a relatively low cost. However, this method is discontinuous and may therefore be subject to improvement, especially for the treatment of sheep and goat hides and thin hides in general, for which it is not necessary to use a vacuum drier. Moreover, the wet stretching of the hides is performed in a single pass, not exploiting to the full the technological and qualitative potential of this process.
Lastly, the use of a vacuum drier makes it necessary to employ a certain number of operators who increase the time and cost of processing.